Recent advances in the development of semiconductor and organic light-emitting diodes (LEDs and OLEDs) have made these solid-state devices suitable for use in general illumination applications, including architectural, entertainment, and roadway lighting, for example. As such, these devices are becoming increasingly competitive with light sources such as incandescent, fluorescent, and high-intensity discharge lamps.
An advantage of LEDs is that their turn-on and turn-off times are typically less than 100 nanoseconds. The average luminous intensity of an LED can therefore be controlled using a fixed constant-current power supply together with pulse width modulation (PWM) of the LED drive current, wherein the time-averaged luminous intensity is linearly proportional to the PWM duty cycle, as illustrated in FIG. 1, wherein three duty cycles of 25%, 50% and 100% are illustrated. This technique is disclosed in U.S. Pat. No. 4,090,189 and documented in Gage, S., M. Modapp, D. Evans, and H. Sorenson. 1977, Optoelectronics Applications Manual, New York, N.Y.: McGraw-Hill Book Company, wherein a single LED colour was considered.
According to W. Howell in a web document “A Brief History of LED Lighting”, Middelsex, UK: Artistic License Inc., 2002, J. Laidman developed a commercial product for a company called Sound Chamber that employed a PWM-based controller for a multiplicity of single-colour LEDs in 1979. A similar PWM-based control method and apparatus also employing a multiplicity of single-colour LEDs was later disclosed in U.S. Pat. No. 4,845,481. According to these inventions, an essentially infinite range of colours can be produced by optically blending single colours of different luminous intensities.
Today, PWM is typically the preferred method for LED luminous intensity control in that it offers linear control over a range of three decades (1000:1) or more without suffering power losses through current-limiting resistors, uneven luminous intensities in LED arrays, and noticeable colour shifts (Zukauskas, A., M. S. Schur, and R. Caska, 2002, Introduction to Solid-State Lighting. New York, N.Y.: Wiley-Interscience, p. 136). The PWM signals used to control the LEDs are preferably generated by microcontrollers and associated peripheral hardware.
There are however, several practical difficulties with implementing PWM control signals in hardware. For example, most microcontrollers offer one to four dedicated PWM channels on-chip, wherein this is usually adequate for individual light fixtures or luminaires that utilize a multiplicity of LEDs with three or four colours, for example red, green, blue, and occasionally amber, whose luminous intensities must be controlled on a per-colour basis. However, some applications may require more PWM channels to control individual LEDs or groups of LEDs.
One example application that can require this form of control is a luminaire where the individual LEDs may be viewed directly. Current LED manufacturing processes result in individual LEDS that exhibit a wide range of luminous intensities for a given constant drive current. LED manufacturers ameliorate this problem by “binning” or sorting LEDs with similar performance characteristics, including luminous intensity. However, the range of intensities within each bin can typically be 30 percent (e.g., Lumileds Lighting, 2002, Application Brief AB22—Luxeon Product Binning and Labeling, San Jose, Calif.: Lumileds Lighting, LLC). Visually critical applications may require a luminaire manufacturer to select LEDs with matching intensity characteristics from within a single bin. Alternatively, the luminaire manufacturer can control the intensity of each LED independently using PWM techniques, however, in this alternative each LED or LED group then requires an independent PWM channel.
Another example application is architectural cove lighting, where a linear array of closely spaced LEDs is used to illuminate a wall adjacent to a ceiling. The length of such an array may range from a few meters to tens of meters. It would be economically advantageous to control the luminous intensities of these LEDs individually or in groups, but with a minimal number of microcontrollers and associated inter-processor communications hardware.
Yet another example application requiring control of individual LEDs or groups thereof occurs when a multiplicity of single-colour LEDs are arranged in a linear array or other geometric pattern wherein it is desired to dynamically change the luminous intensities of individual LEDs or LED groups in order to effect varying patterns of colour and/or luminous intensity. These types of applications can include, for example entertainment lighting systems commonly known as “marquee” or “chase” lighting.
Separate PWM controller integrated circuits (ICs) with up to 48 independent channels that communicate with microcontrollers are commercially available for the above identified purposes. Examples of these ICs include the LD71D1048 PWM controller (Logic Device Technology, 2003, LD71D1048—48 Output LED Driver/10 Bit PWM Controller (Product data sheet)) the MIC5400 LED video display driver (Micrel, Inc. 2002, MIC5400—Dual, 8-Output, 14-Bit LED Video Display Driver (Product data sheet)), and the SL70D0948 PWM controller (System Logic Semiconductor SL70D0948—48 Output LED Driver/9 Bit PWM Controller, (Product data sheet)). Alternatively, it may also be possible to implement a custom PWM controller using field-programmable gate arrays (FPGAs) or application-specific integrated circuits (ASICs) for high-volume production, for example.
These devices have two common problems, however. First, they are physically large devices with up to 128 pins, wherein this feature makes it difficult to route the PWM channel signals to the LED drivers without resorting to, for example expensive multilayer printed circuit boards. And secondly, the devices typically have limited heat dissipation capabilities, which may require additional line drivers for the PWM channel signals if the LEDs are remotely located from the PWM controller.
As an example, PWM control signals can be implemented in firmware using general-purpose microcontrollers. However, the PWM signal frequency should typically be at least 100 Hertz or preferably higher in order to avoid visually perceptible flicker associated with the illumination produced by the LEDs. As such, this requirement typically makes it impractical to implement more than one or two channels on a microcontroller (typically with a CPU clock frequency of 20 MHz) in the absence of a dedicated hardware timer for each channel.
There is therefore a clearly identifiable need for a simple hardware circuit design that can generate a multiplicity of control signals, and which does not require expensive multilayer circuit boards to route the control signals to the LED drivers.
In the fields of voice and data communications, a well-known alternative to PWM is pulse-code modulation (PCM). This technique was originally developed for voice telephony applications and disclosed in U.S. Pat. No. 2,272,070. In its original form, an analog input signal is regularly sampled and represented by digital code. Unlike PWM however, there is no linear relationship between the average on-time of the digital code and the analog input signal. Consequently, it would appear that there are no apparent advantages to the use of conventional PCM techniques in controlling LED drive currents.
There is however, a variant of PCM that can be considered competitive with PWM for the control of LEDs. In a Web document, “Application Note 011: An Overview of the Electronic Drive Techniques for Intensity Control and Colour Mixing of Low Voltage Light Sources Such As LEDs and LEPs”, 2002, W. Howell proposed what he called “bit angle modulation,” or BAM. He described this technique as, essentially driving an LED “by a pulse train that is the binary word defining the value of the required intensity. Each bit of the pulse train is stretched by a ratio defined by the binary significance of the bit.” Comparative examples of output signals using PWM and BAM are illustrated in FIG. 2, wherein each identifies sixteen discrete signal levels.
Howell noted that BAM is most efficient in terms of microcontroller resources with the following example: “A microprocessor generating an eight-bit resolution PWM signal at 100 Hz will need to process the output every 39 μsec, a total of 256 times per output cycle. By comparison, a microprocessor generating an eight-bit resolution BAM signal at 100 Hz will need to process the output only 8 times at 5000 μsec, 2500 μsec, 1250 μsec, 625 μsec, 312 μsec, 156 μsec, 78 μsec and 39 μsec intervals from the cycle start.” He asserted that this represents an 800 percent reduction in required processing power compared to PWM.
As described, BAM can be useful for implementation using microcontrollers that do not have hardware PWM channels, and also communicate with a host controller or perform other tasks in addition to controlling the LED drivers, for example. It does not however address the problems of independently controlling a multiplicity of LEDs with a single microcontroller. While it is true that a firmware BAM implementation requires much less processing power than would an equivalent PWM implementation, the microcontroller must still respond to a hardware timer whose shortest duration pulse was 39 μsec in Howell's example. Assuming a typical microcontroller instruction cycle time of 200 nsec (using a 20 MHz clock), it would be difficult to control more than 20 or so independent BAM channels.
In addition, having regard to Howell's example it appears somewhat optimistic, as most LED drivers for lighting applications require ten-bit resolution at 200 Hz in order to avoid visually perceptible flicker when dimming LED intensities. This typical requirement therefore reduces the minimum BAM pulse width to 5 μsec, and the number of BAM channels to perhaps four for the same 20 MHz clock. (This assumes that the microcontroller needs to devote most of its processing time budget to servicing the hardware timer interrupts and controlling the BAM output signals.)
If for example, a single microcontroller could independently control 128 LED driver channels, it would be desirable to limit the number of physical control lines from the microcontroller to perhaps eight or sixteen, representing one or two eight-bit digital output ports. Therefore, there is a clear need for an apparatus and method that can control a large number of LEDs or group of LEDs wherein physical connections between the source microcontroller and the LED drivers is reduced.
This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.